When I wrote my load and balance Excel sheet for the 737 I had exactly that problem with the trim setting. All the other parameters were either linear or could be adequately interpolated from a look-up table that was not too complicated. I knew there are programmes that will calculate the formula required for the complex curve you face, but I could not find one. I got very close (acceptably close) by a process of 'ball parking' a complex equation by doing what you have done - plotting the points on a graph as AAG suggests but then using one of the graph drawing progs to try and approximate to the function required. Once I got close to the curve, a bit of tweaking using a few sample points did the job. Good luck.
There must be a clever way!